Biaxially stretched polypropylene films are used for electronic and electrical devices. Due to their excellent electrical properties, such as voltage resistance and low dielectric loss, as well as their high moisture resistance, these films are widely used as dielectric films for capacitors, such as high-voltage capacitors; filter capacitors, including various switching power supplies, converters, and inverters; and smoothing capacitors.
In recent years, capacitors have been required to have a smaller size and a higher capacitance. In order to increase the capacitance of a capacitor without changing the volume of the capacitor, it is necessary to reduce the volume of the film, that is, to make the film thin. Therefore, thinner films are required.
Further, polypropylene films are beginning to be widely used as capacitors for inverter power supplies that control drive motors of electric cars, hybrid cars, etc.
Capacitors for inverter power supplies used in cars etc. are required to have a small size, a light weight, a high capacitance, and long-term high voltage resistance (i.e., maintenance of capacitance) in a wide temperature range of −40° C. to 90° C.
For example, PTL 1 discloses that a polypropylene film with a high isotacticity can be formed by controlling the isotacticity and stereoregularity of the polypropylene film within a specific range that has high stereoregularity. PTL 1 also discloses that the heat shrinkage factor of a biaxially oriented polypropylene film is normalized using suitable film-forming conditions to improve dielectric breakdown resistance at a high temperature, and that a capacitor that is prevented from undergoing long-term degradation at a high temperature can be obtained (see claim 1 and paragraph [0013] of PTL 1).
Further, PTL 2 discloses a biaxially oriented polypropylene film having a base layer with pearskin-like irregularities on at least one surface of the film and having specific surface properties such that the ten-point average roughness (Rz) of the surface is 0.50 to 1.50 μm, and the surface gloss is 90 to 135%, wherein the film has excellent processing suitability even when the film is thin, and the film has high voltage resistance in a temperature range of −40° C. to 90° C. (see claim 1 and paragraph [0020] of PTL 2).
Generally speaking, however, if stereoregularity increases, crystallinity also increases, and stretchability decreases; therefore, the film is easily broken during stretching, and high stereoregularity is not preferable in terms of production. Furthermore, PTL 1 and PTL 2 are not sufficient for recent extremely severe requirements for capacitors.
In order to reduce the thickness of the film, it is also necessary to increase the stretchability of the polypropylene resin and cast sheet. However, increased stretchability generally conflicts with improvement of voltage resistance due to increase in stereoregularity and crystallinity, as stated above.
PTL 3 discloses a cast sheet using a polypropylene resin having a specific weight average molecular weight, a specific molecular weight distribution, and a specific stereoregularity, wherein the sheet has a β-crystal fraction that is controlled within a relatively low specific range. PTL 3 also discloses that a biaxially stretched film with a thin film thickness having excellent processing suitability and high voltage resistance can be produced from this sheet (see claim 1, paragraphs [0001] and [0015], etc., of PTL 3). However, there is room for improvement in order to satisfy recent severe requirements relating to voltage resistance at high temperatures.
PTL 4 discloses that a thin polypropylene film having high voltage resistance, without having high stereoregularity, can be obtained by causing the film to have a relatively high crystallinity, to contain a specific molecular weight component, and to have a specific molecular weight distribution due to the component. However, PTL 4 is silent about recent severe requirements relating to voltage resistance at high temperatures.
PTL 5 discloses a polypropylene sheet with a high β-crystal fraction using, as a raw material, a polypropylene resin having a specific melt flow rate, specific Mn, specific Mw/Mn, and specific Mz/Mn, as well as having a specific stereoregularity index. Effects relating to β-crystal formation and moldability have been improved by adjusting the melt flow rate etc. within a specific range; however, there is room for improvement in order to satisfy recent high heat resistance and high voltage resistance.
PTL 6 discloses a polypropylene film with a high β-crystal fraction produced from a polypropylene resin having a broad molecular weight distribution using a simple one-step method. However, PTL 6 is still not sufficient to obtain a thin film having higher voltage resistance and satisfying recent severe requirements.
Since the capacitor industry is rapidly progressing, there is a demand for smaller capacitors, that is, even thinner films. In addition, there is a demand for polypropylene films having more excellent initial voltage resistance and being usable for a longer period of time, that is, excellent long-term voltage resistance. However, existing capacitors do not satisfy these demands.